1. Field of the Invention
The present invention relates to a system for sending and receiving data of the type where frames, each divided into a supervisory data field and an information data field in data format, are successively transmitted, in particular a synchronous multiplex transmitted, in particular a synchronous multiplex transmission apparatus, and more particularly relates to a receiving unit in such a synchronous multiplex transmission apparatus.
2. Description of the Related Art
Multiplexing of digital data on a transmission line is an essential technique for the efficient use of a transmission medium and economical signal transmission.
In particular, the synchronous multiplexing system proposed in CCITT Recommendation G.707, G.708, and G.709 multiplexes 50 Mbps unit data in frame units to enable transmission of several Gbps of data and enables realization of high speed, large volume data transmission.
In such a synchronous multiplexing system, as explained in further detail later, the data is transmitted in units of frames, each frame consisting of a fixed length supervisory data field and a fixed length information data field. A fixed length unit of information data is accommodated in the afore-mentioned information data field. The head position of the information data is indicated by a pointer in the supervisory data field.
Further, the CCITT recommendation defines a stuffing reception in consideration of the slight deviation between the clock frequency on the transmission side of the data and the clock frequency on the reception side due to, for example, temperature fluctuations on the transmission line. In this stuffing reception, there is a so-called "negative stuff" wherein part (byte 1 byte) of the supervisory data field is allocated to part of the information data and a so-called "positive stuff" wherein unnecessary stuff bits (1 byte), that is, dummy bits, are added as information data to the information data field.
In the above-mentioned synchronous multiplex transmission, in the state where no stuff is generated, the pointer indicates the head position of the information data, but when the stuff is generated, information on the type of the stuff, i.e., which of the negative stuff or positive stuff is generated, is indicated by the pointer. Therefore, when a stuff is generated, it is necessary to find the head position of the information data one frame before and the head position of the information data within the current frame in accordance with the type of the stuff generated. Currently, there is a demand for a synchronous multiplex transmission system wherein it is possible to easily detect the head positions.
In particular, as explained later, when a negative stuff is generated, there are cases in which the head positions of two information data fields are simultaneously present in a single frame, so it is impossible to find and specify the head position of the information data effectively. Further, when a positive stuff is generated, there are cases in which no head position of the information data exists at all in the frame, so it also is impossible to find and specify the head position of the information data effectively.
In a synchronous multiplex transmission apparatus, in particular in the receiving unit, one of the important operations is finding and specifying of the head positions of the information data from among the successive frames being transmitted in to and received by the receiving unit. The reason is that unless the head position of the information data can be specified, it is impossible to extract only the information data selectively from the successive frames.